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In a D1 (DECOUPLED MOLDING 1) process the first stage is filling and packing, yet a D2 process isolates filling in the first stage. Could you please explain the difference?

A reverse profile is typically used when processing materials such as nylon and LCP. In fact it is quite common for many semi-crystalline polymers. Basically, such polymers have a melting point. The high rear temperature often helps improve conveyance as well as melt the polymer quickly, Once melted, the polymer can often be cooled a little in the front to improve the overall cycle time and meet the desired melt temperature.

Usually the need for a forward or reverse temperature profile should be approached scientifically. The middle barrel temperatures should be set to the desired melt temperature. The front zones are typically adjusted to obtain the desired melt temperature. The rear temperature should be set  to optimize feed and melt consistency.

Historically, low viscosity polymers with high crystallinity tend to process best using a reverse profile… but you should always look to optimize this, since everyone has different screws, barrel, heater bands, thermocouples, and temperature controllers.

Try not to confuse the front temperature with the actual nozzle temperature. The nozzle temperature will be adjusted independently to meet the needs of the specific process, such as avoiding drool or freeze off.

-Andy

Basically, there are two areas within the molding process where back-flow typically occurs:

1) Transfer From Fill To Pack: If the mold is completely full during first stage, the mold will begin to pressurize as the machine tries to continue pack the mold with first stage fill. When the machine transfers to pack using a lower pressure, a little amount of the pressurized material within the mold will tend to back-flow. In some cases, this back flow will cause sinks or voids on the molded part… in other cases, you may see delamination occur. If you are using a pressure monitoring system, you may see a sharp drop in cavity pressure at the point where the machine transfers from fill to pack, especially near the gate.

2) At The End Of Pack: Insufficient packing time results in a gate which is not completely sealed. As a result, some of the pressure in the mold cavity will become relieved by a small back-flow of material across the gate into the runner system. This most often results in sinks or voids near the gate area. If you are using a pressure monitoring system, you should see a significant drop in pressure at the cavity pressure sensor near the gate when the packing pressure times out.

Keep in mind… if your process is properly established, you should notice a nice gradual rise and fall of pressure within the mold cavity and little to no back-flow within the mold cavity.

-Andy